Narrowband filters increase contrast in emission nebulae by only allowing a specific wavelength range of light to pass through around the spectral lines of hydrogen (H-a, 656 nm), oxygen (OIII, 501 nm), sulphur (SII, 672 nm), etc. The filters can always be used for photography, even when the moon is in the sky, or as filters against light pollution in a large city. The length of the exposure time is irrelevant – even long-exposure photographs can be taken.
The narrow band range of wavelengths is defined as FWHM (full width at half maximum intensity). Narrower band filters reduce background noise. This makes narrow-band filters much more difficult and expensive to manufacture, as attention must be paid not only to the wavelength range but also to the transmission in this small wavelength range. The most difficult part is achieving high transmission for only a very small wavelength range. If, for example, the transmission curve were to drop, the signal for the wavelength range would also drop and the gain for the S/N (signal-to-noise ratio) would no longer be achieved.
This high-quality light pollution filter can be used visually and photographically to increase contrast and allows observations of nebulae, galaxies and star clusters even in light-polluted areas.
